This invention generally relates to air recovery batteries, and cathodes for these batteries.
Batteries are commonly used electrical energy sources. A battery contains a negative electrode, typically called an anode, and a positive electrode, typically called a cathode. The anode contains an active material that can be oxidized; the cathode contains an active material that can be reduced. The anode active material is capable of reducing the cathode active material. In order to prevent direct reaction of the anode material and the cathode material, the anode and the cathode are electrically isolated from each other by a separator.
When the battery is used as an electrical energy source in a device, electrical contact is made to the anode and the cathode, allowing electrons to flow through the device and permitting the respective oxidation and reduction reactions to occur to provide electrical power. An electrolyte in contact with the anode and the cathode contains ions that flow through the separator between the electrodes to maintain charge balance throughout the battery during discharge.
Air recovery batteries, also known as an air assisted or air restored batteries, are batteries in which the cathodes of the batteries are recharged by air. Air recovery batteries can have anodes including zinc powder, cathodes including manganese dioxide (MnO2), and an aqueous solution of potassium hydroxide as the electrolyte.
At the anode, zinc is oxidized to zincate:Zn+4OH−→Zn(OH)42−+2e−.
At the cathode, MnO2 is reduced to manganese oxyhydrate:MnO2+H2O+e−→MnOOH+OH−.
When the cell is not in use or when the rate of discharge is sufficiently slow, atmospheric oxygen enters the cell and reacts with the cathode, thereby recharging the MnO2 cathode:½O2+MnOOH→MnO2+OH−.
During high rates of discharge, air recovery batteries operate like conventional alkaline cell by reducing “fresh” (unreduced) MnO2. During low rates of discharge and periods of rest with no current flow, the “consumed” (reduced) MnO2 is restored or recharged by atmospheric oxygen to the fresh state.
The cathode must be wetted with the electrolyte for the reduction of MnO2 to occur. However, the cathode of the battery should not be completely wetted by electrolyte because oxygen must reach the MnO2 for recharging. If the cathode is completely wetted with electrolyte, air transport properties inside the cathode can be degraded, and the recharging of MnO2 can slow down or stop completely.